Translate all nio SDK calls to mautrix equivalents while preserving the
adapter structure, business logic, and all features (E2EE, reactions,
threading, mention gating, text batching, media caching, voice MSC3245).
Key changes:
- nio.AsyncClient -> mautrix.client.Client + HTTPAPI + MemoryStateStore
- Manual E2EE key management -> OlmMachine with auto key lifecycle
- isinstance(resp, nio.XxxResponse) -> mautrix returns values directly
- add_event_callback per type -> single ROOM_MESSAGE handler with
msgtype dispatch
- Room state (member_count, display_name) via async state store lookups
- Upload/download return ContentURI/bytes directly (no wrapper objects)
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